Light amplifier with non-linear response to provide improved contrast characteristics



Oct. 3, 1967 Y D. R. CHARLES LIGHT AMPLIFIER WITH NON-LINEAR RESPONSE T D CONTRAST CHARACTERISTIC IMPROVE Filed Jan. 29, 1964 United States Patent 3,345,534 LIGHT AMPLIFIER WITH NON-LINEAR RE- SPONSE TO PROVIDE IMPROVED CONTRAST CHARACTERISTICS Daniel R. Charles, Paris, France, assignor to CSF-Compagnie Generale de Telegraphic Sans Fil, Paris, France Filed Jan. 29, 1964, Ser. No. 340,876 Claims priority, application France, Feb. 15, 1963, 924,994, Patent 1,356,459 13 Claims. (Cl. 315-12) The present invention relates to a method for amplifying light as well as to apparatus for realizing such method, and more particularly relates to a method and apparatus for simultaneously increasing the brightness and contrast of an optical image.

Known in the prior art are various types of light amplifiers which transform images having low brightness, either in visible or infra-red light, into images having a stronger brightness. As these prior art amplifiers have generally a linear response, the contrast of the image remains unchanged, which renders very difficult the utilization of images with weak or very weak contrast as, for example, those of objects illuminated by nocturnal light or by a very weak infra-red radiation.

To improve the contrast at the same time as the brightness, it is necessary that the light amplifier have a nonlinear response, and more particularly a quadratic or square-law response.

The present invention has as its object means and a method making it possible to realize a substantially quadratic or square-law light amplification.

According to the present invention, one transforms a light image into another more bright and more contrasted light image by means of an electron tube comprising a luminescent screen, a photocathode capable of emitting photoelectrons toward the screen, a storage grid made of dielectric material and supported by a metallic grid, placed into the path of the electrons, and focusing and accelerating electrodes disposed between the photocathode and the storage grid, the said dielectric material being selected among those having a high degree of secondary electron emission, or those susceptible of becoming conductors in the course of the passage of fast electrons by the effect of induced conductivity.

The method utilized according to the present invention consists of:

uniformly charging the entire surface of the storage grid in such a manner that all of the points of the dielectric have a common negative potential V with respect to the photocathode;

projecting the light image onto the photocathode;

carrying the photocathode at a potential such that the illuminated points of the surface thereof emit fast electrons causing a discharge of the different points of the storage grid by a quantity substantially proportional to the light intensity i of the points corresponding to the image in such a manner that the potential of these points becomes V+v with v-ki, k being a constant;

commutating or shifting the photocathode to a potential such that the illuminated points of the surface thereof emit slow electrons traversing the interstices of the storage grid with an intensity i of which the value is a function, in each interstice, of the value corresponding to the product vizki and accelerating the electrons after the passage thereof across the storage grid in such a manner that the impact thereof forms on the luminescent screen a new image of which the intensity varies with the intensity of the accelerated electrons i, and, consequently, with the square of the initial image.

According to one aspect of the present invention, the

discarge of the storage grid takes place by secondary electron emission.

According to another aspect of the present invention, the discharge of the storage grid takes place by the phenomenon of induced conductivity.

Accordingly, it is an object of the present invention to provide a method and apparatus for amplifying light which obviates the shortcomings encountered with the prior art methods and apparatus by extremely simple means.

It is another object of the present invention to provide a method for amplifying light which permits an amplified electron image with increased brightness and contrast.

Still another object of the present invention resides in the provision of a non-linear light amplifier which is simple in structure, easy to operate yet assures a non-linear amplification of the light to enable improvement in the contrast thereof.

Still a further object of the present invention resides in the provision of a non-linear light amplifier for use in connection With the amplification of images having very weak contrasts such as those obtained by night light or very weak infra-red radiation.

These and other obejcts, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, one embodiment in accordance with the present invention.

Referring now to the drawing which shows in schematic longitudinal cross section a light amplifier in accordance with the present invention, the light amplifier illustrated therein comprises, within an evacuated envelope 1, a photocathode 2, focusing electrodes 3 and 4, an accelerating electrode in the form of a grid 5, a storage grid of dielectric material 6 carried by a metallic support grid 7, and a metallized fluorescent screen 8.

The dielectric material constituting the storage grid is selected among those furnishing a secondary electron emission with a coefficient 6 1 when they are bombarded by fast primary electrons, or those susceptible of becoming conductors of the electricity under the bombardment of fast electrons, or still satisfying both of these conditions at the same time. Among the latter are known, for example, magnesium fluoride or zinc fluoride.

Since the interstices of the dielectric grid 6 must rigorously face those of the support grid 7, one may utilize a grid of fine metallic wires carrying a coating made of the selected dielectric material.

Voltage sources, schematically illustrated and represented at S S and S and a voltage divider 9 make it possible to carry the various electrodes at the desired potentials. Voltage values showing, for illustrative purpose only, the orders of magnitude utilized with the present invention are indicated in the drawing.

A commutator or switch 10 with three positions which may be either manually or automatically controlled, for example, by conventional electronic control means, makes it possible to carry the photocathode 2 at three different potentials that are suitably chosen.

A source 11 of rectangular pulses of short duration is connected across a condenser C to the metallic support grid 7, itself connected to ground across a resistance R, while a light source capable of illuminating the entire surface of the photocathode 2 is disposed at 12.

To amplify the brightness and the contrast of a given image, one operates in accordance with the present invention in the following manner:

(a) As the commutator or switch 10 rests on the right terminal (0 volt), one illuminates the photocathode 2 with the aid of the light source 12; the entire emissive surface of the photocathode 2 then emits uniformly slow electrons toward the storage grid 6. The storage grid 6 emits secondary electrons with a coefificient 5 1 and consequently becomes negatively charged by taking the potential of the cathode.

One applies thereupon to the metallic support grid 7 the short positive pulse from the source 11 which may be, for example, about +10 volts; at the end of this pulse, the potential of the entire dielectric 6 finds itself set uniformly to a negative value with respect to the cathode, to 1() volts in the given example.

(b) One extinguishes thereupon the light source 12 and one projects onto the photocathode 2, if desired, across a lens 14, the light image to be intensified, represented in the figure by the arrow 13, and one places the commutator or switch 10 into the intermediate position, carrying in the figure the indication 2 kv. or to the left position, carrying in the drawing the indication -6 kv., depending on whether one chooses one or the other of the two following variations:

First variation-Discharge by secondary emission The commutator 10 is placed in the middle position bearing the indication 2 kv. The photocathode 2 finds itself in that case at a potential sufficient in order that the illuminated points of its surface emit fast electrons. Under the impact of these fast electrons, the storage grid 6 becomes the seat of a secondary electron emission with a coefiicient 6 l. The various charged points of the dielectric are thus discharged by a quantity proportional to the light intensity of the corresponding points of the image, which takes place by the formation, on the storage grid, of an electrostatic image constituted by a charge pattern corresponding to the light image 13.

In effect, if one calls V the initial negative potential of the storage grid 6, (V=l0 volts in the example under consideration), this potential changes for the various points of the dielectric into V+v, the value of v being for each point substantially proportional to the electron current i which arrives thereat from the cathode, vwki, k=constant.

Second variati0n.Dischar-ge by induced conductivity If the storage grid 6 is made of a dielectric material susceptible of becoming a conductor of electricity under the bombardment of fast electrons, in accordance with a phenomenon known under the name of induced conductivity, the photocathode 2 is connected to the left position of the commutator or switch 10, hearing the indication of -6 kv. in the drawing, and the discharge of the various points of the storage grid 6 takes place by the flow in the metallic support grid 7. The electrostatic image in the form of a charge pattern is then obtained by this phenomenon of induced conductivity in the place of secondary electron emission, the relation v-ki being valid in both cases.

(c) While the image 13 is still projected onto the photocathode 2 and the light source 12 is extinguished, one brings the potential of the photocathode 2 back to 0 volt by moving the commutator or switch 10 to the right position thereof. The illuminated points of the photocathode 2 then emit slow electrons which traverse the interstices of the storage grid 6 after which they are accelerated and reach the luminescent screen 8.

The interstices of the storage grid 6 play, under these conditions, the same role as those of the control grid in a triode amplifier. The different interstices in the present case have different electrostatic potentials, determined by those of the dielectric which surrounds the same. Consequently, the intensity i of the electronic current traversing an interstice varies with the value of the product vi, and as v-ki and vi=ki it follows that i varies as i The image produced on the luminescent screen 8 has then a light intensity which varies as the square of the light intensity of the initial image.

Owing to this substantially square or quadratic amplification, the images obtained are at the same time more bright and have more contrast than those which may be obtained by the known light amplifiers of the present state of the art.

While I have shown and described one embodiment in accordance with the present invention, it is understood that the same is not limited thereto, but is susceptible of numerous changes and modifications within the spirit and scope thereof as known to a person skilled in the art. For example, the amplifier described herein may be utilized in circuits utilizing several tubes in cascade. It suflices for that purpose to dispose the equipment in such a manner that the luminescent screen of one tube projects its image onto the photocathode of the next following tube. However, one may also unite the luminescent screen of one stage and the photocathode of the next following stage into a single plate carrying a cathode-luminescent deposit.

Thus, while I have shown and described only one embodiment, it is obvious that the present invention is not limited thereto but is susceptible of numerous changes and modifications, and I therefore do not wish to be limited to the details shown and described herein =but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

I claim:

1. A non-linear light amplifier tube comprising:

fine mesh storage grid means provided with an insulating layer on one side thereof and with a conductive layer on the other side thereof, a photocathode and a luminescent screen facing said insulating and conductive layers, respectively,

focusing and accelerating electrode means positioned between said photocathode and said storage grid means,

means for projecting onto said photocathode an optical image,

means for successively energizing said photocathode with relatively high voltage and with relatively low voltage electrical energy for successively converting said optical image into a fast electron image and a slow electron image, respectively,

said fast electron image producing on said storage grid a charge pattern representative of said optical image while said slow electron image is amplified under the control of said charge pattern, with the resulting amplified electron image impinging on said luminescent screen thereby producing thereon an optical image with increased brilliance and contrast.

2. A non-linear light amplifier tube comprising:

fine mesh storage grid means provided with an insulating layer on one side thereof and with a conductive layer on the other side thereof,

said insulating layer exhibiting high secondary electron emission properties when struck by fast electrons, a photocathode and a liminescent screen facing said insulating and conductive layers, respectively,

focusing and accelerating electrode means positioned between said photocathode and said storage grid means,

means for projecting onto said photocathode an optical image,

means for successively energizing said photocathode with relatively high voltage and with relatively low voltage electrical energy for successively converting said optical image into a fast electron image and a slow electron image, respectively,

said fast electron image producing on said storage grid a charge pattern representative of said optical image while said slow electron image is amplified under the control of said charge pattern, with the resulting amplified electron image impinging on said luminescent screen thereby producing thereon an optical image with increased brilliance and contrast.

3. A non-linear light amplifier tube comprising:

fine mesh storage grid means provided with an insulating layer on one side thereof and with a conductive layer on the other side thereof,

said insulating layer exhibiting induced conductivity properties when bombarded by fast electrons,

a photocathode and a luminescent screen facing said insulating and conductive layers, respectively,

focusing and accelerating electrode means positioned between said photocathode and said storage grid means,

means for projecting onto said photocathode an optical image,

means for successively energizing said photocathode with relatively high voltage and with relatively low voltage electrical energy for successively converting said optical image into a fast electron image and a slow electron image, respectively,

said fast electron image producing on said storage grid 2. charge pattern representative of said optical image while said slow electron image is amplified under the control of said charge pattern, with the resulting amplified electron image impinging on said liminescent screen thereby producing thereon an optical image with increased brilliance and contrast.

4. A non-linear light amplifier tube comprising:

storage grid means provided with an insulating layer,

a photocathode and a luminescent screen,

focusing and accelerating electrode means positioned between said photocathode and said storage grid means,

means for projecting onto said photocathode an optical image,

means for successively energizing said photocathode with relatively high voltage and with relatively low voltage electrical energy for successively converting said optical image into a fast electron image and a slow electron image, respectively,

said fast electron image producing on said storage grid a charge pattern representative of said optical image while said slow electron image is amplified under the control of said charge pattern, with the resulting amplified electron image impinging on said luminescent screen thereby producing thereon an optical image with increased brilliance and contrast.

5. An electron image intensifying apparatus comprising, within an evacuated envelope:

a fine mesh storage grid means provided with an insulating layer on one side thereof and with a conductive layer on the other side,

a photocathode and a liminescent screen facing said insulating and conductive layers, respectively,

focusing electrodes and an accelerating electrode positioned between said photocathode and said storage grid means,

means for selectively applying a relatively low and a relatively high potential difference between said accelerating electrode and said photocathode,

means for substantially uniformly illuminating said photocathode during a time t while applying said low potential diflerence to cause a substantially uniform emission of slow electrons flooding the insulating surface of said grid means and establishing thereon a substantially uniform potential,

means for applying to the conductive layer of said grid means a voltage pulse during a time t for shifting said uniform potential to a negative value with respect to said photocathode,

and means for projecting onto said photocathode an optical image during a time t While successively applying said relatively high and low potential differences for successively converting said optical image into a fast electron image and a slow electron image, said fast electron image producing on said storage grid a charge pattern corresponding to said optical image, said slow electron image being amplified under the control of said charge pattern by virtue of triode action in which the particular meshes of said storage grid means serve as triode control grids and said luminescent screen serves as a plate, the amplified electron image impinging on said luminescent screen and producing thereon an optical image exhibiting increased brilliance and contrast. 6. An electron image intensifying apparatus comprising within an evacuated envelope:

storage grid means, a photocathode, a luminescent screen, accelerating electrode means, means for selectively applying a relatively low and a relatively high potential difference between said accelerating electrode means and said photocathode,

means during a time t for establishing a substantially uniform potential on said storage grid means,

means for applying to said grid means a voltage pulse during a time t for shifting said uniform potential to a negative value with respect to said photocathode,

means for projecting onto said photocathode an optical image during a time t While successively applying said relatively high and low potential differences for successively converting said optical image into a fast electron image and a slow electron image, said fast electron image producing on said storage grid means a charge pattern corresponding to said optical image, said slow electron image being amplified under the control of said charge pattern by virtue of triode action in which said storage grid means serves as triode control grids and said liminescent screen serves as a plate, the amplified electron image impinging on said luminescent screen and producing thereon an optical image exhibiting increased brilliance and contrast.

7. A method of converting a first optical image into a second optical image with increased brilliance and contrast by means of an electronic storage tube including a fine mesh storage grid positioned between a photocathode and a luminescent screen, said method consisting of the steps of:

fixing said storage grid at a negative potential with respect to said photocathode, converting said first optical image by means of said photocathode into a fast electron image impinging on said storage grid to produce thereon a charge pattern representative of said first optical image,

and converting said first optical image by means of said photocathode into a slow electron image propagating through the meshes of said storage grid toward said luminescent screen, thereby obtaining an amplified electron image producing on said luminescent screen said second optical image.

8. The method as defined in claim 7 wherein said step of fixing said storage grid at a negative potential with respect to said photocathode is accomplished by uniformly illuminating said photocathode to produce a flood of slow electrons impinging upon said storage grid and subsequently appyling to said storage grid a voltage pulse for shifting the potential thereof to a negative value with respect to said photocathode.

9. The method as defined in claim 7 wherein said step of converting said first optical image into a fast electron image is accomplished by projecting said first optical image on said photocathode while applying thereto at the same time a first bias voltage having a relatively high value sufiicient to effect emission of fast electrons from the illuminated points of the photocathode surface.

10. The method as defined in claim 9 wherein said first 'bias voltage has a value sufficient to effect emission of sufliciently fast electrons to produce induced conductivity properties in said storage grid.

11. The method as defined in claim 9 wherein said first bias voltage has a value suflicient to effect emission of fast electrons sufficiently only to produce high secondary emission properties in said storage grid.

12. The method as defined in claim 9 wherein said step of converting said first optical image into a slow electron image is accomplished by switching said photocathode to a second bias voltage having a relatively low value sufficient only to effect emission of slow electrons from the illuminated points of the photocathode surface.

13. A method of converting a first optical image into a second optical image with increased brilliance and contrast by means of an electronic storage tube having at least a fine mesh storage grid positioned between a photocathode and a luminescent screen and a source of potential, said method consisting of the steps of:

uniformly charging the entire surface of the storage grid until all of the points of the dielectric have a common negative potential V with respect to the photocathode,

projecting said first optical image onto the photocathode,

applying a first bias voltage to the photocathode having a relatively high value suflicient to effect emission of fast electrons from the illuminated points of the photocathode surface causing a discharge of the different points of the storage grid by a quantity substantially proportional to the light intensity i of the points corresponding to the image such that the potential of the points becomes -V+v with vzki, k being a constant, then switching said photocathode to a second bias voltage having a relatively low value sufiicient only to efiect emission of slow electrons from the illuminated points of the photocathode surface which traverse the interstices of the storage grid with an intensity i of which the value is a function, in each interstice, of the value corresponding to the product vizki and accelerating the electrons after passage thereof across the storage grid in such a manner that the impact thereof on the luminescent screen forms an image having an intensity which varies with the intensity of the accelerated electrons i References Cited UNITED STATES PATENTS 2,875,360 2/1959 Kruper 31-365 X 2,903,596 9/1959 Reed 250-213 2,992,346 7/ 1961 Farnsworth 3l5-11 X 2,992,358 7/1961 Farnsworth 31511 3,002,101 9/ 1961 Anderson 250-213 3,243,643 3/1966 Toohig 315-12 JOHN W. CALDWELL, Acting Primary Examiner.

ROBERT L. GRIFFIN, Examiner.

T. A. GALLAGHER, R. L. RICHARDSON,

Assistant Examiners. 

1. A NON-LINEAR LIGHT AMPLIFIER TUBE COMPRISING: FINE MESH STORAGE GRID MEANS PROVIDED WITH AN INSULATING LAYER ON ONE SIDE THEREOF AND WITH A CONDUCTIVE LAYER ON THE OTHER SIDE THEREOF, A PHOTOCATHODE AND A LUMINESCENT SCREEN FACING SAID INSULATING AND CONDUCTIVE LAYERS, RESPECTIVELY, FOCUSING AND ACCELERATING ELECTRODE MEANS POSITIONED BETWEEN SAID PHOTOCATHODE AND SAID STORAGE GRID MEANS, MEANS FOR PROJECTING ONTO SAID PHOTOCATHODE AN OPTICAL IMAGE, MEANS FOR SUCCESSIVELY ENERGIZING SAID PHOTOCATHODE WITH RELATIVELY HIGH VOLTAGE AND WITH RELATIVELY LOW VOLTAGE ELECTRICAL ENERGY FOR SUCCESSIVELY CONVERTING SAID OPTICAL IMAGE INTO A FAST ELECTRON IMAGE AND A SLOW ELECTRON IMAGE, RESPECTIVELY, SAID FAST ELECTRON IMAGE PRODUCING ON SAID STORAGE GRID A CHARGE PATTERN REPRESENTATIVE OF SAID OPTICAL IMAGE WHILE SAID SLOW ELECTRON IMAGE IS AMPLIFIED UNDER THE CONTROL OF SAID CHARGE PATTERN, WITH THE RESULTING AMPLIFIED ELECTRON IMAGE IMPINGING ON SAID LUMINESCENT SCREEN THEREBY PRODUCING THEREON AN OPTICAL IMAGE WITH INCREASED BRILLIANCE AND CONSTRAST. 